This application is a U.S. National Phase application of PCT International application PCT/JP99/06688.
The present invention relates to non-aqueous electrolyte secondary batteries and charging methods of the same, and especially relates to non-aqueous electrolyte secondary batteries (hereinafter, batteries) with high energy density, whose electrochemical properties such as charge/discharge capacity and charge/discharge cycle life have been enhanced by improvements of negative electrode materials and non-aqueous electrolytes.
In recent years, lithium secondary batteries with non-aqueous electrolytes, which are used in such fields as mobile communications devices including portable information terminals and portable electronic devices, main power sources of portable electronic devices, small size domestic electricity storing devices, motor cycles using an electric motor as a driving source, electric cars and hybrid electric cars, have characteristics of a high electromotive force and a high energy density.
The lithium ion secondary batteries with an organic electrolytic solution, which use carbon materials as negative electrode active materials and lithium-containing composite oxides as positive electrode active materials, have higher voltage and energy density, and superior low-temperature properties compared with secondary batteries using aqueous solutions. As these lithium ion batteries do not use lithium metal in the negative electrode, they are superior in terms of cycle stability and safety, thus are now being commercialized rapidly. Lithium polymer batteries using macromolecular (polymer) gel electrolytes which contain an organic electrolytic solution, have been also under development as a new thin and light batteries.
When lithium metal with a high capacity is used as a negative electrode material, dendritic deposits are formed on the negative electrode during charging. Over repeated charging and discharging, these dendritic deposits penetrate through separators and polymer gel electrolytes to the positive electrode side, causing an internal short circuit. The deposited dendrites have a large specific surface area, therefore their reaction activity is high. Thus, it reacts with plasticizers (solvents) of the polymer gel electrolytes, lowering charge/discharge efficiency. This raises the internal resistance of the batteries, causing some particles to be excluded from the network of the electronic conduction, thereby lowering the charge/discharge efficiency of the battery. Due to these reasons, the lithium secondary batteries using lithium metal as a negative electrode material have a low reliability and a short cycle life.
Nowadays, lithium secondary batteries, which use, as a negative electrode material, carbon materials capable of intercalating and de-intercalating lithium ions, are commercially available. In general, lithium metal does not deposit on negative electrodes with carbon. Thus, short circuits are not caused by dendrites. However, the theoretical capacity of graphite which is one of the currently used carbon materials is 372 mAh/g, only one tenth of that of pure Li metal.
Other known negative electrode materials include pure metallic materials and pure non-metallic materials which form compounds with lithium. For example, composition formulae of compounds of tin (Sn), silicon (Si) and zinc (Zn) with the maximum theoretical amount of lithium are respectively Li22Sn5, Li22Si5, and LiZn, and within the range of these composition formulae, metallic lithium does not normally deposit. Thus, an internal short circuit is not caused by dendrites. Furthermore, electrochemical capacities between these compounds and each element in its pure form mentioned above is respectively 993 mAh/g, 4199 mAh/g and 410 mAh/g; all are larger than the theoretical capacity of graphite.
As other compound negative electrode materials, the Japanese Patent Laid-open Publication No. H07-240201 discloses a non-metallic silicide comprising transition elements. The Japanese Patent Laid-open Publication No. H09-63651 discloses negative electrode materials which are made of inter-metallic compounds comprising at least one of group 4B elements, phosphorus (P) and antimony (Sb), and have a crystal structure of one of the CaF2 type, the ZnS type and the AlLiSi type.
However, the foregoing high-capacity negative electrode materials have the following problems.
Pure metallic and pure non-metallic materials used as negative electrode materials, and which form compounds with lithium, commonly have inferior charge/discharge cycle properties compared with carbon negative electrode materials. The reason for this is assumed to be destruction of the negative electrode materials caused by volume expansion and contraction.
On the other hand, as negative electrode materials with improved cycle life properties, unlike the foregoing pure materials, the Japanese Patent Laid-open Publication No. H07-240201 and the Japanese Patent Laid-open Publication No. H09-63651 respectively disclose non-metallic silicides composed of transition elements and inter-metallic compounds composed of at least one of group 4B elements, phosphorus (P) and antimony (Sb), and with a crystal structure of one of the CaF2 type, the ZnS type and the AlLiSi type.
Batteries with negative electrode materials comprising non-metallic silicides composed of transition elements, and disclosed in the Japanese Patent Laid-open Publication No. H07-240201, have improved charge/discharge cycle properties compared with lithium metal negative electrode material, in terms of the capacities of embodiments of the invention and a comparative example at the first cycle, the fiftieth cycle and the hundredth cycle. However, when compared with a natural graphite negative electrode material, the increase in the capacity of the battery is only about 12%.
The materials disclosed in the Japanese Patent Laid-open Publication No. H09-63651 have a better charge/discharge cycle property than a Li-Pb alloy negative electrode material according to an embodiment and a comparative example. The materials also have a larger capacity compared with a graphite negative electrode material. However, the discharge capacity decreases significantly until the 10-20th charge/discharge cycles. Even when Mg2Sn, which is considered to be better than any of the other materials, is used, the discharge capacity decreases to approximately 70% of the initial capacity after about the 20th cycle. Thus, they are inferior in terms of charge/discharge properties.
Regarding charging methods for these batteries, the Japanese Patent Laid-open Publication No. H06-98473 discloses a method in which a pulse current is added while charging a lithium secondary battery in order to restrict dendritic deposits of lithium on a lithium negative electrode. As for lithium ion secondary batteries whose negative electrodes comprise carbon materials, the Japanese Patent Laid-open Publication No. H04-206479 discloses a method in which the level of a charging current is controlled so as to be under a predetermined level during charging at a constant voltage in order to prevent lithium dendrites from depositing on a carbon negative electrode.
However, the charge/discharge cycle life properties vary depending on the charging method for the batteries. The predominant reason for this is as follows; since oxidation reduction potential during charging and values of over-voltage during electrochemical reaction are different, if charging current or voltage exceed predetermined tolerances, electrode reactions proceed unevenly, or other side reactions such as deposits of lithium, formation of film or generation of gas occur, thus lowering the charge/discharge cycle life properties.
The present invention aims to address the foregoing problems of the conventional batteries.
The negative electrode of the batteries of the present invention is characterized by its main material which uses composite particles constructed in such a manner that at least part of the surrounding surface of nuclear particles containing at least one of tin, silicon and zinc as a constituent element, is coated with a solid solution or an inter-metallic compound which is composed of,
the element included in the nuclear particles and
at least one other element (exclusive of the elements included in the nuclear particles) selected from a group comprising group 2 elements, transition elements, group 12 elements, group 13 elements and group 14 elements (exclusive of carbon) of the Periodic Table.
The present invention is further characterized by one of the following conditions:
the lithium content of the nuclear particles of the composite particles is 40-95 atomic percent of the theoretical limit of lithium content of each constituent element of the nuclear particles, namely, tin, silicon and zinc;
the lithium content in the composite particles is 50-90 atomic percent of their theoretical limit of lithium content; and
when the negative electrode, exclusive of the current collector, contains an amount of lithium which allows no lithium to be electro-deposited, the volume expansion of the negative electrode exclusive of the current collector is 110-200%.
Further, according to the present invention, the batteries whose negative electrode comprises the composite particles, are charged at a predetermined constant current (I) until they reach a predetermined voltage (E), and upon reaching the predetermined voltage (E), charged at the predetermined constant voltage (E). The values of the current (I) and the current during the constant voltage charging are set at not more than 5 mA/cm2 as a current density in the area where the positive and negative electrodes face each other.
With the foregoing construction, the problems associated with the conventional batteries are solved, thus providing non-aqueous electrolyte secondary batteries and their charging methods achieving high-energy density and superior cycle life properties.